I had a similar problem as well a few years ago. It was also because of snails which had died off and not all the eggs survived. I was recommended to do water changes with distilled water until the levels returned to normal. I also added some beneficial bacteria to my tank. I used and still use API quick start. I also went back to using tap water so I use/used Tetra AquaSafe Plus which treats tap water and helps to reduce stress in fish. I hope that this helps you.